1. Field of the Invention
The present invention relates to an optical waveguide device to be used for optical communications, and in particular, relates to a Mach-Zehnder type optical modulator.
2. Description of the Related Art
For example, an optical waveguide device in which an electro-optic crystal such as lithium niobate (LiNbO3), lithium tantalite (LiTaO2) or the like is used, is manufactured by forming a metal film on a part of a crystal substrate to perform thermal diffusion or proton exchange in benzoic acid after patterning, thereby forming an optical waveguide, and afterwards providing electrodes in the vicinity of the optical waveguide. A Mach-Zehnder type optical modulator with a branching interference type optical waveguide structure is known as one of optical waveguide devices using such electro-optical crystal.
FIG. 6 is a plan view showing the structure of a typical Mach-Zehnder type optical modulator. In this Mach-Zehnder type optical modulator, an optical waveguide 110 is formed such that a titanium (Ti) film is formed on a substrate 101, patterned in a Mach-Zehnder shape, and afterwards heated for seven to ten hours at 1050° C. to be subjected to the thermal diffusion. This optical waveguide 110 comprises an incoming waveguide 111, a branching section 112, parallel waveguides 113A and 113B, a multipplexing section 114, and an outgoing waveguide 115. A coplanar electrode 120 comprising a signal electrode 121 and a ground electrode 122 is provided along the parallel waveguides 113A and 113B. In the case where a Z-cut substrate 101 is used, in order to utilize a change in refractive index due to an electric field in a Z direction, the signal electrode 121 is positioned just above the optical waveguide 113A. Further, the signal electrode 121 and the ground electrode 122 are formed on the substrate 101 via a buffer layer (not shown in the figure) formed of silicon oxide (SiO2) or the like, in order to prevent the absorption of lights which are propagated through the parallel waveguides 113A and 113B.
In the case where such a conventional Mach-Zehnder type optical modulator is driven at a high speed, one end of the signal electrode 121 is grounded via a resistor (not shown in the figure) to be made a traveling-wave electrode, and a high frequency electrical signal M, such as a microwave or the like, is applied from the other end of the signal electrode 121. At this time, since the refractive indexes of the parallel waveguides 113A and 113B are both changed due to an electric field generated between the signal electrode 121 and the ground electrode 122, phase differences of the lights being propagated through the parallel waveguides 113A and 113B are changed, and an intensity modulated optical signal is output from the outgoing waveguide 115.
For the above described conventional Mach-Zehnder type optical modulator, it has been known that by changing the cross-section of the signal electrode 121 to control the effective refractive index of microwave, and matching propagation speeds of the light and the microwave, broadband optical response characteristics can be obtained. Furthermore, as shown in FIG. 7 and FIG. 8 for example, by providing a curved section 110A in a part of the optical waveguide 110 to form an approximately U-shaped or S-shaped optical waveguide 110, it is possible to achieve the miniaturization (especially, miniaturization in the longitudinal direction) of the substrate 101.
In the case where the curved section is formed in the part of the optical waveguide, an optical loss (radiation loss) occurring in the curved section is a problem. Conventionally, a technique has been proposed for reducing the loss in the curved section of optical waveguide, in which a reflecting section is disposed on an outer periphery of the curved section, to again couple the light emitted from the curved section in the optical waveguide (refer to Japanese Unexamined Patent Publication No. 11-167032).
However, the above described conventional Mach-Zehnder type optical modulator has the following problems.
(1) Problem Regarding Wavelength Chirp
In the conventional Mach-Zehnder type optical modulator, since the intensity of the electric field applied to each of the parallel waveguides 113A and 113B differs from each other depending on a difference in arranged positions of the parallel waveguides 113A and 113B relative to the signal electrode 121, a change amount (Δns) of the refractive index of the parallel waveguide 113A, which is closer to the signal electrode 121, is greater than a change amount (Δng) of the refractive index of the parallel waveguide 113B, which is further from the signal electrode 121. Therefore, absolute values of the phase changes of the lights being propagated through the parallel waveguides 113A and 113B differ from each other. Thus, there is a problem in that when the signal is switched from “0” to “1” or from “1” to “0”, a wavelength change (wavelength chirp) in a modulated light occurs, which degrades the signal waveform after transmission.
In order to reduce the wavelength chirp, for example there is a method in which an X-cut crystal substrate is used, or a method in which two signal electrodes are arranged on the respective parallel waveguides to push-pull drive the Mach-Zehnder type optical modulator.
In the case where the X-cut crystal substrate is used, by applying electric fields to the two parallel waveguides in a +Z direction and a −Z direction utilizing an electric field parallel to the substrate, it becomes possible to perform modulation whereby the wavelength chirp does not occur. However, since it is not possible to arrange the parallel waveguides directly below the signal electrode, there is a large distance between the signal electrode and the waveguides. Therefore, there is a disadvantage in that a high drive voltage must be applied.
Furthermore, in the case where the push-pull drive is performed using two signal electrodes, two input connectors for high frequency electrical signals are required, and also electrical signals with their data inverted must be applied to the two signal electrodes, while their phases being controlling. Therefore, there is a disadvantage in that the circuit structure of drive system becomes complicated.
(2) Problem Regarding Miniaturization
In order to miniaturize the Mach-Zehnder type optical modulator, even if the curved section is formed in the part of the optical waveguide, and furthermore, the optical loss occurring in the curved section is suppressed using the above described technique of Japanese Unexamined Patent Publication No. 11-167032, it is difficult to reduce the curvature of the curved section of the optical waveguide. Therefore, there is a problem in that miniaturization of the optical modulator is limited. For example, in the case where the curvature of the curved section of the optical waveguide is made small as approximately a few mm, the lights are not again coupled sufficiently by the reflecting section, and hence the optical loss is increased significantly. In the case where the curvature of the curved section of the optical waveguide cannot be made small in this manner, it is difficult to make the whole of the optical waveguide small, which leads to a limitation to the miniaturization of optical modulator.
Here, for the above described problem (1), the inventor of the present application has proposed a technique for reducing the wavelength chirp occurring in the modulated light, by forming a polarization inversed region in a part of a portion where the lights being propagated through the parallel waveguides and the microwave being propagated through the signal electrode interact, arranging the signal electrode above one of the parallel waveguides in the polarization inversed region, and arranging the signal electrode above the other parallel waveguide in a non-inversed region, and thereby offsetting the difference between the amounts of phase change of the lights being propagated through the two parallel waveguides (refer to a prior application, International Application No. PCT/JP02/12824). However, this prior invention has not show a specific configuration in which the curved section is formed in the part of the optical waveguide as described above, and a problem remains regarding the miniaturization of optical modulator.
Further, for the above problem (2), a technique has been proposed for miniaturizing an optical device, by lowering areas of the substrate on both sides of the curved optical waveguide along the curved section shape to form a ridge structure section, and forming a buffer layer on at least a side face of the ridge structure section that faces the curved section by the use of a material whose refractive index is lower than the refractive index of the substrate, so that it is possible to reduce the radiation loss or the like even if the curvature of the curved section is small (refer to a prior application, Japanese Patent Application 2003-079116,). However, this prior invention leaves a problem regarding the above described wavelength chirp. Moreover, by forming the ridge structure section, the substrate under the signal electrode has a side face inclined diagonally. Therefore, there is a problem in that the design for matching the speeds of the lights being propagated through the optical waveguides and the speed of the microwave being propagated through the signal electrode becomes complicated.